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The Tawallah Valley Meteorite. General Description. the Microstructure.Records of the Australian Museum 21(1): 1–8, Plates I–Ii

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The Tawallah Valley Meteorite. General Description. the Microstructure.Records of the Australian Museum 21(1): 1–8, Plates I–Ii AUSTRALIAN MUSEUM SCIENTIFIC PUBLICATIONS Hodge-Smith, T., and A. B. Edwards, 1941. The Tawallah Valley meteorite. General description. The Microstructure.Records of the Australian Museum 21(1): 1–8, plates i–ii. [4 July 1941]. doi:10.3853/j.0067-1975.21.1941.518 ISSN 0067-1975 Published by the Australian Museum, Sydney nature culture discover Australian Museum science is freely accessible online at http://publications.australianmuseum.net.au 6 College Street, Sydney NSW 2010, Australia THE TAW ALLAH VALLEY METEORITE. General Description. By T. HODGE-SMI'l'H, The Australian Museum. The Microstructure. By A. B. EDWARDS, Ph.D., D.I.C.,* Research Officer, Mineragraphy Branch, Council for Scientific and Industrial Research. (Plates i-ii and Figures 1-2.) General Description. Little information is available about the finding of this meteorite. Mr. Heathcock, Constable-in-Charge of the Borroloola Police Station, Northern Territory, informed me in April, 1939, that it had been in the Police Station for eighteen months or more. It was found by Mr. Condon, presumably some time in 1937. The weight of the iron as received was 75·75 kg. (167 lb.). A small piece had been cut off, but its weight probably did not exceed 200 grammes. The main mass weighing 39·35 kg. (86i lb.) is in the collection of the Geological Survey, Department of the Interior, Canberra. A portion weighing 30·16 kg. (66~ lb.) and five pieces together weighing 1·67 kg. are in the collection of the Australian Museum, and a slice weighing 453 grammes is in the Museum of the Geology Department, the University of Melbourne. The locality is Tawallah Valley, about forty-eight miles north-west of Borroloola, Northern Territory, Australia, latitude 15° 42' S.,longitude 135° 40' E. approximately. The shape of the iron is quite unusual tn that it is more or less fiat and there is an almost complete absence of thumb·marks so characteristic of meteoric iron. The iron contains only a few very small inclusions of troilite and no schreibersite has been found. This does support the view that thumb-marks are due to the decomposition of such minerals during the m'eteorite's fiight through the atmosphere. The fiat side forms roughly a parallelogram the longer sides of which are very straight and almost parallel. They are 65 cm. in length and approximately 38 cm. apart. The smaller sides of the parallelogram forming the head and tail of the meteorite are not so straight. The tail particularly is curved toward the centre. The thickness gradually decreases from the head (65 mm.) to the tail (5 mm.). There is a general tapering from one side to the other, though this difference in thickness disappears toward the tail. One wing of the tail is bent at an angle of about 25° to the plane of the iron. It is possible that this was bent when the iron struck the ground, as there is a fairly large oval indentation on the edge just where the bending takes place, and there is no doubt that this indentation was formed"mechanically. The reverse side is almost perfectly fiat with a number of indentations that have been mechanically formed. The larger ones appear to be the result of the iron striking something hard and sharp on reaching the ground. Their formation and orientatioln confirm the fact that the thick end of the iron was actually the head during flight. The smaller indentations are not oriented and resemble chisel marks amd may have been made by man. * Published by permission of the Council for Scientific and Industrial Research. A 2 RECORDS OF THE AUSTRALIAN MUSEUM. The obverse side is not so fiat. There is a mound rising about 25 mm. above the level surface, situated about half-way between the head and the tail and close to the thicker side. There are also four depressions so arranged as to give the appearance of the foot-mark of some beast. There are chisel-like marks similar to those on the reverse side amd, like them, probably the result of human activity. The outer skin is black and very thin. In a number of places it is worn through, showing the bright iron underneath. There is no sign of rippling of the surface similar to that of the Bugaldi (New South Wales) iron. Around the head on the fiat side only is a layer of iron less than 1 mm. in thickness, extending not more than 40 mm. from the periphery and ending in a very irregular edge. This may possibly represent the remnant of a wave of molten metal leaving the forward end. On the corner of the head and on the thinner side are two large oval indentations mechanically formed, probably at the time of cOhtact with the ground. The first cutting was kindly undertaken by the Chief Mechanical Engineer's Branch of the New South Wales Railways Department. It was cut by a slitting saw in a milling machine. Later, when it was found that the iron was soft and comparatively free from inclusions, a complete section was cut by hand at the Museum. The Railway Department did the polishing. The iron takes a brilliant polish and does not rust easily. Ohemioal Analysis. Fe Ni Co Pt S P C Total Sp. Gr. n 82·29 16·90 1·09 Trace abs. abs. 0·03 100·31 8·00 4'8 The absence of sulphur would indicate the complete absence of troilite, but this can be true only for the portion analysed, as there are a few scattered inclusions of troilite in a complete section of the iron. The largest of these inclusions does not exceed 2 mm. in diameter. No schreibersite has been detected. The very small amount of carbon is present, in part at least, in a very finely divided state. Of the platinoid metals no iridium was detected and platinum was found to be present to the extent of 0·003 per cent. The specific gravity is somewhat high. Three separate samples were tested by the ordinary chemical balance method, giving as result 8'002, 8'002, 8'001. Another piece tested by the Chemical Branch of the Department of Mines gave a result of 7·997. The average of these readings is 8'00. This is the highest value that I have been able to find for any me.teorite except for the Botetourt iron (0. Sjostrom, 1898), of which the specific gravity is recorded as 8·186. The theoretical speGific gravity for an iron-nickel of the composition of the TawalIah Valley, using the specific gravities given in the Smithsonian Physical Tables (1920), is for the minimum value 7·903 and for the maximum 8'001. On etching a polished surface with dilute nitric acid a distinct etch pattern was produced showing in its orientation a resemblance to the usual WidmansUitten figures. The usual bands of kamacite and taenite were replaced by a definite sheen. A similar etch pattern has been recorded for many of the nickel-rich irons. No satisfactory explanation of this structure has been offered. Dr. Edwards very kindly offered to undertake a mineragraphic examination with a view to supplying this explanation. That he has been entirely successful will be realized from his' notes published below. According to the analysis, this iron belongs to the nickel-rich ataxites, Group 3 of Prior's Classification. Spencer records twenty-three nickel-rich ataxites. Since that time the Tschinga iron (Ni 16'71 per cent; n 5) has been recorded, so that this iron makes the twenty-fifth record and the first for Australia. The ArItunga (Central Australia), with a value for n of 8'6, is just on the border-line between the finest octahedrites and the nickel-rich ataxites. It is reported as having a micro-octahedral structure. The Yarroweyah (Victoria) is the only Australian nickel-poor ataxite. In view of the apparently constant structure to be found in nickel-rich iron and theiil fact, as shown by Dr. Edwards for the first time, that this structure is closely related THE TAWALLAH VALUW METEORITE-HODGE-SMITH AND EDWARDS. 3 to the octahedral structure, the name ataxite, meaning without order, is no longer applicable. The term ataxite should be confined to those irons in which the nickel content is too low to produce a definite structure of, the nickel-iron alloys. It is suggested that the term eotaxite should be applied to those irons rich in nickel in which the early stage of the formation of the WidmansUitten structure is developed. The Microstructure. Examination of the polished sections of the Tawallah Valley iron in reflected light reveals that it consists almost wholly of nickel-iron with an occasional bleb of iron sulphide, from 1 to 2 mm. in diameter. Iron Sulphides. The blebs of iron sulphide vary somewhat in composition. Some consist of troilite, others of pyrrhotite. The distinction between them is that troilite, which contains more sulphur, effervesces with dilute nitric acid, and gives off hydrogen sulphide, which stains the iron, while the pyrrhotite is practically inert to dilute nitric acid. One bleb of pyrrhotite examined consisted of several allotriomorphic crystals, some of which showed lamellar tWinning. The pyrrhotite is creamy-brown in colour, strongly anisotropic, and shows reflection pleochroism. The powder is magnetic. Of the standard etching reagents, potassium hydroxide slowly stains the pyrrhotite brown, while the other reagents are negative. When a portion of the meteorite was dissolved in dilute nitric acid, a complete, but somewhat etched, prismatic crystal of pyrrhotite, about 2 mm. x 0·5 mm., was set free.
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